1. Field of the Invention
The present invention relates to a technology for calculating a toner density.
2. Description of the Related Art
A variety of image forming apparatuses use toner to form images, i.e., they form toner images. Example of such image forming apparatuses are analog image forming apparatuses, digital image forming apparatuses, black-and-white copiers, color copiers, printers, plotters, facsimile machines, and, multifunction printers (MFPs).
To form a good quality toner image, as is widely known, an electrostatic latent image needs to be developed with just an appropriate amount of toner. The electrostatic latent image can be developed with a two-component developer that contains toner and carrier or a single-component developer that contains only toner. An amount of the toner to be supplied to a developing unit that develops the electrostatic latent image is called, hereinafter, “toner density”.
If the toner density is low, i.e., if the amount of the toner supplied to the electrostatic latent image is less than the necessary amount, a paler toner image will be formed. If the toner density is high, i.e., if the amount of the toner supplied to the electrostatic latent image is more than the necessary amount, a darker and difficult-to-see toner image will be formed. To form a good quality toner image, the toner density should be within an appropriate range.
To adjust the toner density to a value within the appropriate range, it is necessary to measure the current toner density. In a typical method, the toner density is measured from a change in a detection light reflected from a toner image that is formed dedicated to the toner-density measurement (hereinafter, “toner pattern”). An optical device that emits the detection light to the toner pattern and receives the detection light reflected from the toner pattern is called a reflective optical sensor.
Various types of reflective optical sensors are known in the art (see Japanese Patent Application Laid-open No. S64-35466, Japanese Patent Application Laid-open No. 2004-309292, Japanese Patent Application Laid-open No. 2004-21164, and Japanese Patent Application Laid-open No. 2002-72612).
Typical reflective optical sensors include a light-emitting unit and a light-receiving unit. The light emitting unit includes one, two, or three light-emitting elements having different wavelength characteristics. The light-receiving unit includes one or two light-receiving elements (e.g., photodiodes (PDs) or phototransistors).
Light-emitting diodes (LEDs)) are typically used as the light-emitting elements. The LEDs emits the detection light of a spot size that is smaller than the toner pattern on the toner pattern.
The toner pattern is formed, for example, on a transfer belt. The toner pattern moves as the transfer belt rotates. A direction in which the transfer belt moves due to the rotation is called a sub-direction, and a direction perpendicular to the sub-direction is called a main-direction. In a system in which electrostatic latent images are formed through optical scanning, the main-direction corresponds to the main-scanning direction, and the sub-direction corresponds to the sub-scanning direction.
An electrostatic latent image corresponding to a toner pattern is formed on a photosensitive member by optically scanning a surface of the photosensitive member with an electrostatic-latent-image forming unit, and the electrostatic latent image on the surface of the photosensitive member is then developed into the toner pattern. The toner pattern on the photosensitive member is then transferred onto the transfer belt, and is moved in the sub-direction with the rotation of the transfer belt. When the toner pattern enters a detection area, the toner pattern is exposed with a spot of the detection light from the reflective optical sensor. The spot size of the spot of the detection light is typically about 2 millimeters (mm) to 3 mm.
In an ideal situation, the spot falls on the center of the toner pattern in the main-direction. However, it is difficult to always keep a relative position between the toner pattern and the reflective optical sensor in the main-direction the ideal state, due to various reasons. These reasons include fluctuation in an optical scanning area of the electrostatic-latent-image forming unit, meandering of the transfer belt, positional shift of the reflective optical sensor in the main-direction from an initial installation position because of passage of time.
If a portion of the spot falls in a region where there is no toner pattern because of the positional miss-match in the main-direction between the toner pattern and the reflective optical sensor, the reflected light received by the light-receiving unit represents wrong data, and therefore the measured toner density is wrong. Assume, for example, that one light-emitting element emits one spot of the detection light, one light-receiving element receives the reflected light, and the toner density is calculated from a difference between a specular reflection light and a diffuse reflection light. The light-receiving element is arranged to receive the specular reflection light. If a first portion of the spot falls in a region where there is no toner pattern and a second portion falls on the toner pattern, the first portion of the detection light is reflected specularly while the second portion is reflected diffusely. As a result, in a configuration where the light-receiving element is arranged so as to receive the specular reflection light, as compared to a case where the entire spot falls out of the toner pattern, intensity of the specular reflection light that is received at the light-receiving element decreases due to the generation of the diffuse reflection light. The decrease in the intensity of the specular reflection light can also occur when the toner amount at the toner pattern is low. Therefore, the decrease in the intensity of the specular reflection light is due to low toner amount or miss-match between the spot and the toner pattern is always unclear.
To solve this problem, in the conventional techniques, the toner pattern of a size from about 15 mm to about 25 mm in both the main-direction and the sub-direction is formed so that the spot of the detection light cannot fall out of the toner pattern even in case of the positional miss-match.
In the image forming apparatuses, specifically, the color image forming apparatus, the measurement of the toner density by the reflective optical sensor using the toner pattern is performed to acquire and maintain high image quality as a maintenance activity necessary for an accurate image-forming process. Because the toner-density measurement is performed as the maintenance activity separated from the main activity, i.e., an image-forming process, the image formation cannot be performed during the toner-density measurement.
When the electrostatic latent image to be developed to the toner pattern is written by the optical scanning, time required for the optical scanning is in proportion to the size of the toner pattern. In other words, the larger the toner pattern is, the lower the operating efficiency of the image formation becomes.
Moreover, because a total amount of the toner in the toner container or the like is fixed, as an amount of the toner to be used for the toner pattern increases, an amount of the toner to be used for the main activity, i.e., the image formation decreases, disadvantageously. The larger the toner pattern is, the more the toner is consumed for the toner pattern. In this manner, the conventional toner-density measuring methods have the two disadvantages, i.e., the low operating efficiency and the large toner-consumption amount for the toner pattern.